Milling cutter heads normally support a plurality of cutting tool elements spaced about the circumference of the head in a manner which projects the cutting edge of each of the tools into a common plane. In many well-known designs, such tool elements are in the form of indexable bits formed with a plurality of similar cutting edges, the bits being commonly shaped in the form of triangles, squares, hexagons, etc., with each side comprising a separate cutting edge. Such indexable bits are also designed in disc-shapes with the entire circumference of the disc forming a cutting edge. When one side of such an angular bit or one segment of such a disc-shaped bit becomes dull, the bit is indexed in the cutterhead to provide a new cutting edge. Examples of such cutters are shown in U.S. Pat. Nos. 3,487,535 and 4,212,567.
In most prior art milling cutters, the individual tool bits are mounted directly in the cutterhead (e.g., U.S. Pat. No. 3,378,901) but, in some designs, the bits are first inserted in a holder or nest plate prior to being received into the cutterhead to simplify manufacture of the head and to facilitate truing adjustment of the individual bits relative to each other (e.g., U.S. Pat. No. 3,675,290).
In these prior art cutters, the blades, or the blade/nest plate combinations, are respectively received into individual openings formed in the body of the cutterhead. These openings, generally rectangular in form, are cut directly in the solid annular body that comprises the major portion of the cutterhead, being open to the outer circumference of the body but having the floor, side, and interior walls formed integral with the solid body.
There are two major criteria which must be met by the design of such milling cutters. The first is rigidity, which must be sufficient to prevent vibration, chatter and any loosening of the relatively small inserted cutting tools under expected cutting forces. The second relates to accuracy and the ability to true each of the separate individual cutting tools so that the cutting edges of all of the tools project in a nearly identical track in the cutting plane. To appreciate the difficulty in meeting this second criteria, it must be understood that, within the present limits of commercial practicality, small, indexable cutter bits can only be manufactured with a tolerance of approximately .+-.0.0005 (0.013 mm) while axial accuracy required for finishing work may approach blade-to-blade "truing" tolerances of .+-.0.00025" (0.007 mm). To achieve such accuracies, the cutter design must usually include some mechanism whereby truing adjustments may be made to the position of each inserted cutting tool. Such adjustment is difficult and takes considerable time, thereby increasing costs and reducing production and, in addition, the adjustment mechanism often results in a reduction of rigidity.
Further, as attempts are made to speed up cutting processes, the cutting forces experienced by the tools increase, and it must be appreciated that the difficulty of meeting rigidity and accuracy requirements increases dramatically. Therefore, even though individual cutting tool elements are presently available in carbide or ceramic materials which are capable of cutting under such higher forces, milling speeds have not been increased to take advantage of such new types of cutting materials because of the rigidity and accuracy problems just referred to above.
An important limitation on such increased speed has been created by the radial stresses which tend to cause failure in the solid body of the cutterhead at the intersections of the side and interior walls of the openings which receive the blades or blade/nest plate combinations.